The present invention relates to changing a system input/output (I/O) configuration in a channel subsystem, and more particularly relates to changing the system I/O configuration dynamically such that the change is made with minimal disruption to the normal system operation without a power-on-reset (POR).
Large data processing installations typically have one or more systems (also referred to herein as central processing complexes), each having one or more central processing units (CPU's), each system having one or more attached I/O devices or being attached to other systems. Such systems are typically managed by a control program such as IBM's MVS/ESA operating system.
Since such a system has a high degree of modularity, it is a routine event to add, delete, or exchange I/O devices. A particular I/O configuration must be defined both to the system's hardware elements and operating system software, and a change to an I/O configuration occasioned by the addition, deletion, or change of a channel path, a control unit or an I/O device must be reflected in a change to the hardware and software definitions.
In the past, it has been a matter of some difficulty to change a system's I/O configuration without disrupting active processing of the system. A normal course of events would be to stop processing of work on the system, add, delete, or change the devices by performing the physical connection, disconnection, or reconnection, then POR the hardware and IPL the operating system to reestablish the correct software and hardware definitions, and again start the processing or work.
This process is unacceptably disruptive and expensive in most cases because it requires that the system be idle for a possibly long time period while the reconfiguration is performed. Schemes devised to reduce the impact on the system processing during such configuration changes include over-defining an I/O configuration by including definitions and related control blocks for spare devices which could then be utilized later when new devices are to be added. The number of such reserved definitions is a matter of careful planning and, in some cases, intelligent guess work. In this scheme, space is wasted by the reservation of unused control blocks, this scheme only allows for the addition and not the deletion of device definitions, and certain types of changes, such as correcting incorrect specifications of device type, would still require a system reinitialization. This scheme further requires device pathing information (i.e., channel-to-control-unit-to-device connections) to be specified at I/O configuration definition. If plans change, this device pathing information needs to be respecified and a subsequent POR and IPL is required to reinitialize the I/O configuration definition.
Other schemes provide for the dynamic addition of a device definition without pre-reservation of control blocks, but do not deal with device definition deletion since such a deletion poses the additional problem of the treatment of ongoing system work making use of the device to be deleted. The traditional method of dealing with this situation was to "quiesce" the system (completing ongoing work without starting new work) so that the device could be disconnected, and the device definition deleted, without adversely affecting ongoing processing. Still other schemes change the characteristics of existing device definitions, leaving unresolved the matter of whether or not active work must stop, and how to deal with device additions or deletions.